The present invention is related to the reduction of turbulence in a pipe. More specifically, the present invention is related to the reduction of turbulence with a conditioner so that the calibration coefficient of an ultrasonic flowmeter can be accurately determined using a volumetric displacement prover.
Transit time ultrasonic flowmeters have exhibited excellent repeatability and absolute accuracy in many flow measurement applications. However, characteristics inherent in the nature of their measurements present difficulties when these meters are applied to custody transfer measurements of petroleum products. (A custody transfer takes place when ownership of a batch of a particular product changes. On a small scale, such a transfer takes place at the pump in a gas station.)
It is industry practice in custody transfer measurements to xe2x80x9cprovexe2x80x9d the meter; that is, to establish its calibration accurately, by independent means. Provers are usually devices of fixed and precisely established volume. The time required to deliver the volume of product defined by the prover is accurately measured by timing the transit period of a ball or piston, pushed by the product, from one end of the prover to the other. High-speed diverter valves initiate the prover run and bypass the prover when the ball reaches the end of its travel. The proving operation is synchronized with the operation of the custody transfer meterxe2x80x94the meter to be used to measure the amount of product delivered to a specific customer. The volumetric output measured by the custody transfer meter (in current practice, a turbine or positive displacement meter) during the prover run is compared to the volume of the prover and a meter factor (i.e., a calibration correction) is established.
It is also industry practice to perform a set of several prover runsxe2x80x94five is typicalxe2x80x94to establish the xe2x80x9crepeatabilityxe2x80x9d of the meter factor of the custody transfer meter. Repeatability in the petroleum industry is usually defined as follows: the difference between the high and low meter factors from a set of prover runs, divided by the low meter factor from that set. Repeatability in the 0.02 to 0.05% range is taken as indicating that the custody transfer meter is in good conditionxe2x80x94suitable for use in measuring the volume of the entire batch whose custody is to be transferred. [The batch volume may be hundreds or thousands of times larger than the prover volume.] The average meter factor as determined from the set of prover runs is used for the custody transfer measurement.
Unlike turbine and positive displacement flowmeters, a transit time ultrasonic flowmeter does not measure volumetric flow rate continuously, but instead infers it from multiple samples of fluid velocity. Specifically, the volumetric flow rate is determined from periodic measurements of the axial fluid velocity as projected onto one or more acoustic paths. The path velocity measurements are combined according to rules appropriate to their number and location in the pipe. Many meters employ parallel chordal paths arranged in accordance with a specific method of numerical integration.
The period over which an ultrasonic transit time meter collects a set of velocity measurements (one or more, depending on the number of paths) is determined by the path transit times, the number of paths, and/or the data processing capabilities of the meter itself. For liquid meters, the sample frequency will typically lie between 10 Hz and 1000 Hz.
An ultrasonic flow measurement is thus a sample data system on two counts:
(1) It does not measure the velocity everywhere across the pipe cross section but only along the acoustic paths, and
(2) It does not measure velocity continuously, but instead takes a series of xe2x80x9csnapshotsxe2x80x9d of the velocity from which it determines an average.
Because of these properties, a transit time ultrasonic meter responds to flow phenomena like turbulence differently than other meters commonly used for custody transfer in the petroleum industry. More specifically, the individual flow measurements of transit time ultrasonic meters will be affected by the small scale random (i.e., turbulent) variations in local fluid velocity. These variations are both temporal and spatial, and an ultrasonic instrument must make multiple measurements to determine the true average flow ratexe2x80x94to reduce the random error contributions due to turbulence to acceptable levels. Turbine meters and positive displacement meters, on the other hand, respond to the flow field in the pipe as a whole; integration of the fluid velocity in space and time is inherent in the nature of their responses. Nevertheless, transit time ultrasonic meters are not encumbered by physical limitations like bypass leakage arid friction, and may therefore provide measurement capability over a wider range of velocity and viscosity conditions.
Although the velocity variations due to turbulence are random, multiple samples will only reduce their contribution to measurement uncertainty/repeatabilityxe2x80x94if the time interval over which the samples are taken is long compared to the periods of the low frequency contributors to the turbulence spectrum. Put another way, a transit time ultrasonic flowmeter can only meet petroleum industry expectations of repeatabilityxe2x80x940.02 to 0.05%xe2x80x94if the number of samples of fluid velocity collected during a prover run include a large number of cycles of the lowest significant turbulence frequency. How many cycles? Enough to reduce the RMS contribution of this low frequency turbulence to the meter factor measurement to a level consistent with the repeatability requirements.
The centroid of the turbulence spectrum varies with fluid velocity. Caldon has measured turbulence intensity at Alden Research Laboratories over a range of fluid velocities typical of those encountered in petroleum and petroleum product pipelines. The data indicate a spectrum centered at about 3 Hz at about 4 feet/second. The spectrum is centered at: about 6 Hz at 8 feet/second, while it is centered at 10 Hz at a velocity of 14 feet/second. These frequency data are generally consistent with the turbulence literature. See, for example, xe2x80x9cStructure of Turbulent Velocity and Temperature Fluctuations in Fully Developed Pipe Flowxe2x80x9d, M. Hishida and Y. Nagano, Journal of Heat Transfer February 1979, incorporated by reference herein. This reference and most others on the subject plot the turbulence energy spectrum against the wave number of the turbulence, given by 2xcfx80f/U, where f is the frequency of the turbulence and U is the free stream velocity. The spectrum is expressed as turbulent energy per unit wave number increment. Turbulence intensity is here defined as [∫Tui2dt/T]xc2xd/U, where [∫Tui2dt/T]xc2xd is the root: mean square of the incremental turbulent velocities ui and U is the mean axial velocity. The incremental turbulent velocities uI represent the temporal and spatial departures of local velocities from the mean. T is a time period encompassing all significant turbulent variations.
The magnitude of the intensity measured by Caldon is also consistent with the literature. For a fluid path length roughly equivalent to a diametral path in a 16-inch pipe, an RMS intensity of 1.6% of the mean axial velocity was measured (for the 4 to 14 ft/sec fluid velocity range). This figure is comparable to that measured in much smaller pipes in the previously cited reference.
A 50-foot long pipe prover for pipeline operating at a flow velocity of 5 ft/sec will generate less than 10 seconds worth of flow data for each prover run. If the turbulence intensity is 1.6%, and the spectrum is centered near 4 Hz, about 40 samples of the low frequency turbulent variations will be collected during a prover run (in spite of the fact that the flowmeter might collect over 1000 measurements in the same 4 seconds). Five meter factor measurements from a single path flowmeter in this application would show a repeatability of about 0.6%. Multiple paths will reduce this figure to perhaps 0.3% or 0.4%; nevertheless all the numbers are at least an order of magnitude higher than the expectations for repeatability in the petroleum industry.
This, then, is the problem. Turbulence, such as normally encountered in petroleum pipelines, adversely affects the repeatability of the meter factors for transit time ultrasonic flowmeters, as measured in short duration prover runs. Unless something is done to alter the character of the turbulence, it appears that meter factors measured for ultrasonic flowmeters with conventional provers will not achieve repeatability figures meeting petroleum industry expectations.
It is important to note, however, that the long-term repeatability of transit time ultrasonic meters is entirely compatible with the custody transfer function. That is, if meter factor is established by a test or group of tests of 5 to 7 minutes total duration, the meter factor thus measured will accurately characterize a transfer of batch custody lasting many minutes or hours. Furthermore, the same meter factor will be found to apply to the same product day in and day out. Additionally, it will be found that the meter factors of some ultrasonic meters are insensitive to significant changes in product viscosityxe2x80x94due to changes in temperature, for example. These attributesxe2x80x94not characteristic of many currently used custody transfer metersxe2x80x94make it desirable to find a means to solve the problem of turbulence.
As further background in regard to this problem, accurate measurement of the lift and drag characteristics of wings and other airfoils in wind tunnels requires the elimination of turbulence because, in free flight, there are no disturbances of sufficiently small scale to produce appreciable aerodynamic effects. (The effect of so-called clear air turbulence on aircraft is produced by large-scale variations in air velocity and direction.) Hence, replication of free flight conditions in a wind tunnel requires the elimination of turbulence. In the 1940s, Dryden and Schubauer tested the effects of screens on the turbulence in wind tunnels, and determined that screens can produce substantial reductions in the intensity of the incident turbulence of a flowing air stream. Hugh L. Dryden and G. B. Schubauer, The Use of Damping Screens for the Reduction of Wind Tunnel Turbulence, Journal of the Aeronautical Sciences, April, 1947. Their work also includes a semi-empirical mathematical treatment of the mechanism whereby the turbulence is reduced. In the development of the present invention, the work of Dryden and Schubauer has been drawn upon and expanded, to suit the unique requirements of a transit time ultrasonic flow instrument.
The screens used by Dryden to reduce turbulence covered a range of configurationsxe2x80x94from 18 to 60 mesh, with wire sizes ranging from 0.017 in (for coarser meshes) to 0.0075 in (for finer meshes). In general, the reduction in turbulence brought about by the screens was found to be given by
T1/T0=1/(1+k)n/2xe2x80x83xe2x80x83(2-1)
Where T=turbulence intensity=[⅓(u2+v 2+w2)]xc2xd/U,
the subscript 0 refers to incident conditions, the subscript 1 refers to conditions some distance downstream of the screen or screens, sufficient to allow eddies shedding from the screens themselves to have dissipated,
u, v, and w are the rms components of the turbulent velocity variations along the x, y, and z axes respectively,
U is the mean free stream velocity (i.e., at station 0),
k is the energy loss per unit volume (head loss) through one screen, and
n is the number of screens in cascade.
Using 6 screens in the settling chamber of his wind tunnel, Dryden was able to reduce an incident turbulence of 1.6% to 0.16% a short distance downstream of the screens. He effected a further reduction, by a factor of 6.6 in the test chamber of the wind tunnel, through the use of a convergent nozzle downstream of the settling chamber and leading into the test chamber. (A convergent nozzle is a standard feature of wind and water tunnels. Its function is to provide a uniform, high velocity profile in the test chamber.) The reduction in turbulence was due to the convergent nozzle""s area ratioxe2x80x94also 6.6xe2x80x94which accelerated the main stream velocity without increasing the highly localized turbulence. The present invention is directed to solving the turbulence problem in transit time ultrasonic flowmeters.
U.S. Pat. No. 5,495,872 to Gallagher is involved with xe2x80x9cswirlxe2x80x9d and the conditioner of Gallagher is for the purpose of minimizing swirl to provide an axisymmetric velocity profile. Swirl is very different from turbulence in that swirl is the laminar movement of the flow at significant angles to the longitudinal axis of the pipe. Gallagher refers to angles of 15 degrees to 20 degrees which are reduced by his conditioner to about 2 degrees. Nowhere has Gallagher made any mention for any application of the conditioner with ultrasonic flow meters, nor recognized the relationship of turbulence with ultrasonic signals.
Conditioners to minimize swirl to about 2 degrees and provide for an axisymmetric velocity profile are well known in the art, such as disclosed in U.S. Pat. No. 5,341,848 to Laws, which also teaches for the conditioner to leave the turbulence structure unchanged; and U.S. Pat. No. 5,959,216 to Hocquet. They make no mention for any application with ultrasonic flow meters, nor recognize the relationship of turbulence with ultrasonic signals.
The present invention pertains to an apparatus for determining fluid flow in a pipe. The apparatus comprises an ultrasonic flowmeter adapted to be placed with the pipe for measuring fluid flow in the pipe. The apparatus comprises a turbulence-reducing flow conditioner adapted to be disposed in the pipe through which the fluid flow in the pipe passes and upstream to the flowmeter.
The present invention pertains to a turbulence-reducing flow conditioner for an ultrasonic flowmeter for a pipe. The conditioner comprises a plate having holes and a thickness and ligament between holes that is sufficient to withstand forces on the conditioner from the fluid flow in the pipe. The plate reduces turbulence intensity T of the fluid and increases frequency of residual turbulence of the fluid after the fluid has passed through the conditioner for the meter.
The present invention pertains to a method for determining fluid flow in a pipe. The method comprises the steps of flowing the fluid in the pipe through a turbulence reducing flow conditioner wherein the conditioner reduces turbulence intensity T of the fluid and increases frequency of residual turbulence of the fluid after the fluid has passed through the conditioner. Then there is the step of measuring the fluid flow in the pipe with an ultrasonic flowmeter after the fluid flow has passed through the conditioner.
The present invention pertains to a method for reducing turbulence in a pipe for measuring flow in the pipe. The method comprises the steps of placing a turbulence reducing flow conditioner, which includes the pipe that contains it, that pipe being a fluid tight enclosure. The conditioner reduces turbulence intensity T of the fluid and increases frequency of residual turbulence of the fluid after the fluid has passed through the conditioner located in the path of fluid flow. Next there is the step of connecting an ultrasonic flowmeter to measure the fluid flow in the pipe. Then there is the step of directing flowing fluid through the conditioner and the ultrasonic flowmeter.